With the release of the “First Report and Order”, Feb. 14, 2002, by the Federal Communications Commission (FCC), interest in ultra wide bandwidth (UWB) systems has increased considerably. UWB systems spread information over a wide bandwidth of at least 500 MHz. Due to this spreading operation, a power spectral density is small. Therefore, interference to narrow bandwidth receivers is also small.
Transmitters for low data rate transmitted reference UWB systems (TR-UWB) are described by R. Hoctor and H. Tomlinson, “Delay-hopped transmitted-reference RF communications,” Proceedings of the IEEE Conference of Ultra Wideband Systems and Technologies 2002 (UWBST'02), pp. 265-269, May 2002, N. v. Stralen, A. Dentinger, K. Welles II, R. Gaus, R. Hoctor, and H. Tomlinson, “Delay hopped transmitted reference experimental results,” Proceedings of the IEEE Conference of Ultra Wideband Systems and Technologies 2002 (UWBST'02), pp. 93-98, May 2002, F. Tufvesson and A. F. Molisch, “Ultra-wideband communication using hybrid matched filter correlation receivers,” Proc. IEEE Vehicular Technology Conference (VTC 2004 Spring), Milan, Italy, May 17-19, 2004, and J. D. Choi and W. E. Stark, “Performance of ultra-wideband communications with suboptimal receivers in multipath channels,” IEEE Journal on Selected Areas in Communications, vol. 20, issue 9, pp. 1754-1766, December 2002.
Those low data rate systems relax stringent timing requirements of the impulse radio (IR) systems, M. Z. Win, R. A. Scholtz, “Impulse radio: How it works,” IEEE Communications Letters, 2(2): pp. 36-38, February 1998, and do not need any channel estimation. Channel estimation is a challenging task for coherent UWB receivers, Lottici, A. D'Andrea, and U. Mengali, “Channel estimation for ultra-wideband communications,” IEEE Journal on Selected Areas in Communications, vol. 20, issue 9, pp. 1638-1645, December 2002.
In the prior art, two basic receiver schemes are known, namely rake receiver with matched filters, see Choi et al., and a transmitted reference scheme that uses a pulse correlator, see Hoctor et al., “Delay-hopped transmitted reference RF communications,” IEEE Conf. on Ultra Wideband Systems and Technologies, pp 265-270, 2002.
The rake approach requires channel estimation for the combining of a selected number of multi-path components. Because the receiver structure is fairly complex, only the strongest, or a few of the strongest multi-path components are used to form the decision variable. That means that the receiver does not fully resolve all multi-path components, and the performance is less than ideal due to the inherent channel estimation and combining problem. Increasing the number of rake fingers increases the complexity and cost of the system.
In transmitted reference schemes, pairs of transmitted pulses are used for each data symbol. The first pulse, called the reference pulse, is not modulated by the data symbol. The second pulse, called the data pulse, is modulated by the data symbol. The reference and data pulses are separated by a time delay. The receiver recovers the data symbol by multiplying time-aligned pulses, which results in a large correlation peak. The different peaks all have the same phase. The phase is determined by the value of the data symbol, and therefore, as an advantage, they can be summed by an integrator. This scheme is less complex and is able to combine the energy from different multi-path components without channel estimation. Unfortunately, the output of the multiplier has a very poor signal-to-noise ratio (SNR) due to non-linear operations on noise terms when forming the decision variable and due to the inherent energy loss when transmitting the reference pulse. That results in large noise-times-noise terms that are integrated over the time. The received signal can be passed through a matched filter to reduce the effects of noise-times-noise terms, see Tufvesson et al., or an averaging operation can be performed, see Choi et al. However, overall, the transmitted reference scheme has a worse performance when compared with the ideal rake approach, due to the noise products.
In the prior art, a single pulse type is used for all signals communicated between a given transmitter and receiver. That is, all reference pulses and all data pulses are of the same type, e.g., all are a Gaussian, or alternatively a monocycle Gaussian. By having the same pulse type for all signals, the likelihood of inter-frame interference (IFI) and multi-access interference (MAI) is increased. Therefore, there is a need for a UWB system that decreases IFI and MAI.